1. Field of the Invention
The invention relates to direct thermal-to-electric energy conversion systems for in situ recovery of underground geothermal energy.
2. Description of Prior Art
High temperature geological areas, commonly referred to as geothermal areas, exist throughout the world. Geothermal areas are created by an intrusion of molten material into fractures and/or defects in the earth's crust to a point relatively near the earth's surface. The upwelling molten material (magma) often exists in large chambers, the uppermost portion of which are geologically proximate to the earth's surface. (3 to 20 km).
The magma chambers are fairly stable structures and contain an enormous amount of heat energy. The molten material within the chamber is believed to be at temperatures between 1200.degree. C. and 1800.degree. C.
Heat energy of the magma in a chamber dissipates slowly, if at all, because of the very poor thermal conductive properties of the solid rock materials surrounding it. Recent investigations suggest a convective communication between the chambers and molten material in the earth's asthenosphere. In fact, it has been suggested that the chambers are in reality created by a circulatory plume of molten material. If such is the case, then heat energy dissipating from the chamber by conduction is replenished by the convective mechanisms between the chamber and the asthenosphere. Also poor thermal conduction properties of solid rock materials limits dissipation of the heat energy of the magma chamber to geographically local areas.
Often geological formations containing water occur proximate a magma chamber. In such occurences heat energy diffusing from the chamber heats and/or vaporizes water of such formations. The water vapor then diffuses to the earth's surface through fractures and defects in geological formations thereabove. The diffusing water vapor is believed to be the source of hot springs, geysers and the like. Such areas are commonly referred to as aqueous geothermal areas.
Where there are no water-bearing or aqueous geological formations proximate a magma chamber, heat dissipates by normal conduction through the surrounding solid rock materials. Such geothermal areas are commonly referred to as hot rock or "dry" geothermal areas.
Heat from aqueous geothermal areas has long been used as a source of energy of human societies. For example, the escaping steam or hot water has been used to heat buildings, wash clothes, cook food and the like.
However, because of the local geographic nature of such geothermal energy phenomenon, the total energy potential of geothermal energy sources have not been utilized as a major energy source by modern societies. Specifically, to utilize effectively the energy potential of a geothermal energy source, the thermal energy must be converted into a more distributable form of energy such as electricity.
Existing systems for converting geothermal energy into electricity are hydothermal systems. Specifically, in aqueous geothermal areas, a well is drilled into the formations for producing steam or hot brine. The steam and/or hot brine is then utilized to drive a conventional turbine-driven generator. In some instances, the steam escaping from the geological formation forms the primary fluid driving the generator system. Such areas are commonly referred to as dry steam systems. Typical examples of such dry steam systems are those located in the Larderello area of Italy and the Geysers area of California.
The brine or hot water systems are far more common. In such systems, the hot water or brine is utilized as a secondary fluid to heat primary fluid for driving a turbine generator. A typical example of a brine system is the Salton Sea area in California.
In both the dry steam and brine systems, the escaping fluid is reinjected back into the formation usually via a second well.
For dry geothermal areas, a system of wells is drilled into the formation and a fluid, such as water, is introduced for heating. A second system of wells is drilled into the formation for collecting the heated or vaporized water which is then utilized to drive a turbine generator, and the cooled fluid is recirculated back into the formation via the first system of wells.
Hydrothermal energy recovery systems of geothermal energy sources have limitations. For example, the circulating fluid (water) often dissolves large quantities of minerals and becomes very corrosive. The dissolved minerals also often precipitate out as the fluid escapes or is pumped from underground. The precipitating minerals can effectively plug the well.
The primary disadvantages of a hydrothermal-convective energy recovery system for geothermal energy sources relates to the relatively low operating temperature (250.degree. C. to 600.degree. C.). Because of such low temperatures the resultant steam is usually not sufficiently super-heated for efficiently driving turbine generators.
In addition, temperatures of hydrothermal-convection systems are largely determined by the ability of the involved geological formations to contain the heated fluid. If the formations are "tight" then unlimited expansion of the fluid is prevented and the temperature of the fluid increases. On the other hand, if the formations are not "tight" then expansion occurs lowering the temperature of the fluid. Also substantial heat energy is lost as latent heat of vaporization. In brine systems the temperature is determined by the boiling temperature of the brine.
Another complex problem in recovering energy from geothermal formations is created by the poor thermal conduction properties of solid rock materials. Specifically, an energy recovery system which removes thermal energy from hot rock at a rate greater than the rate of heat flow through the surrounding rock effectively insulates itself from the energy source. More concisely, the rate of heat conduction through the solid rock materials limits the rate of recovery of thermal energy from a geothermal energy source. Accordingly, a geothermal energy recovery system should be optimized for the particular thermal conduction properties of the rock formations within the geothermal region.